"You wait ages for an atheist bus and then 800 come along at once," said Ariane Sherine at the culmination of her campaign to put anti-faith slogans on the sides of UK buses. Throughout, she has been the antithesis of the shrill, dogmatic and shouty atheist that is so beloved of religious caricaturists. She is charming, smiley and wonderfully unintimidating and her campaign has captured the imagination of thousands of people.

Last summer, she was irritated by an advert on a bus from a religious organisation that expressed a profoundly threatening message (essentially, think what we think or "you spend all eternity in torment in hell"). She wondered if there was a classier and more thoughtful way of putting the opposing point of view.

The result – thanks to £135,000 raised mainly through small donations – is a national fleet of 800 buses with the words "There's probably no God, now stop worrying and enjoy your life" on the side, plus a series of adverts on the London Underground. The phrase is a genius of understatement. They are the weary words of so many of us who have had to listen tight-lipped to endless wild-eyed preachers presuming to tell us we were born sinful and are destined for eternal damnation. Those silent millions have suddenly found a voice.

With the low December sun glistening off the gold of the Albert memorial in the background, the atheist bus campaign's press launch felt at times more like a Church of England coffee morning. There were warm greetings, hugs between old friends and purposeful-looking people marching back and forth with clipboards looking stressed about whether the catering would be adequate. The cast was a mini-Who's Who of British atheists: evolutionary biologist Professor Richard Dawkins; the philosopher AC Grayling; president of the British Humanist Association Polly Toynbee; comedian Robin Ince. Even Stephen Fry sent a goodwill message from New Zealand (not enough cash yet for a live video link up).

Ince, who hosted the event, pointed out that the buses were not technically atheist, but agnostic. "That is the scientific viewpoint because we can't ultimately prove that there is no God. But I'm taking the risk," he said.

Dawkins is another man who is taking the risk. "I would have chosen a slightly stronger slogan," he confided, "but I wouldn't have wanted to say there's definitely no God just as I wouldn't want to say there's definitely no Father Christmas."

All in all, Sherine's campaign has been a magnificent and surprising achievement. The original fundraising target of £5500 was reached in just 3 hours – and at its peak, the campaign's Just Giving website was clocking up £17 per minute. The site has continued to attract new donors on every single day since its launch.

For my money, it is Sherine's amusing and charming slogan that has prompted so many people to part with her hard-earned cash. Pledging support was a way for non-believers to make a statement about what they didn't believe – and in the process, to make some other people smile and perhaps think a bit on their way to work. Atheists are by definition free-thinkers who don't follow the crowd. The atheist bus gave us a way of speaking with a single voice – and it feels great. As Sherine put it, "I don't feel alone anymore."